When developing internal combustion engines, the reduction of harmful substances, which are created during the combustion in the internal combustion engine, play an increasingly larger role. On the one hand, suitable exhaust gas purification devices or exhaust aftertreatment devices, such as catalytic converters, may be provided in order to filter the created harmful substances out of the exhaust gas after they have been expelled from the internal combustion engine or to convert them into less harmful products. On the other hand, the object is, however, to prevent the harmful substances from being created in the first place.
One possibility for achieving this object is to provide an exhaust gas recirculation. Here, the exhaust gas of the internal combustion engine created during the combustion is not immediately removed and released into a surrounding, but is resupplied to the internal combustion engine together with fresh air and fuel. For this purpose, the exhaust gas is recirculated to the fresh-air side of the internal combustion engine. In this way, the nitrogen oxide amount contained in the exhaust gas may be reduced. The combustion temperature prevailing in the internal combustion engine is initially reduced due to the exhaust gas recirculation. This is due to the fact that although the exhaust gas does not partake in the combustion itself, it still must be heated during the combustion. Such a reduction in the combustion temperature results in a reduction of the nitrogen oxide (NOx) content. Moreover, the exhaust gas recirculation causes a reduced oxygen content during the combustion. Nitrogen oxides are, however, preferably created if there is excess air, so that the nitrogen oxide content is further reduced. It must be noted, however, that the amount of oxygen present during the combustion may not be too small because hydrocarbons (HC) may otherwise form, which is not desirable either.
The effect of the exhaust gas recirculation may be further improved by cooling the recirculated exhaust gas. In this regard, German Patent Application No. DE 10 2006 055 814 A1, for example, describes an internal combustion engine having a turbocharger (charging device), the exhaust gases of the internal combustion engine being taken from an exhaust gas system and fed into its fresh-air line, so that the fresh air with the exhaust gases mixed in there is compressed in a compressor part of the turbocharger and is supplied to an intake-side inlet manifold. A cooling element is integrated into the inlet manifold and is designed in such a way that the possibly created condensate may only flow into the direction of the intake valves of the internal combustion engine, and the fresh air which is mixed with the exhaust gases is cooled within the inlet manifold, the inlet manifold emptying into an air diffuser. Thus, the exhaust gas is recirculated to the low pressure level before a compressor turbine, in order to be able to bring a larger amount of the exhaust gas to the fresh-air side of the internal combustion engine, and the temperature of the exhaust gas is subsequently reduced, together with that of the fresh air.
Another possibility for recirculating a larger amount of exhaust gas to the fresh-air side of the internal combustion engine is to provide a throttle valve to the fresh-air side or in a fresh-air supply, in order to temporarily reduce the pressure prevailing there, in particular an intake pipe pressure. If the internal combustion engine has a charging device, the recirculated exhaust gas must be compressed together with the fresh air. This means that the charger output must be significantly increased because the additional exhaust gas mass flow is to be taken into account. This also applies when a throttle valve is used, since the charging device must now compensate for the throttle losses. Consequently, the fuel consumption also increases because the intake action increases due to the reduced pressure available to the internal combustion engine.